It is well known to provide a pen having free ink that a user may selectively apply to a substrate such as paper. Such known pens typically include a reservoir for storing the ink and a channel for ducting the ink from the reservoir to a marking tip. The ink of such known pens typically has a vapor pressure such that the ink, and any air in the reservoir, expands and contracts in response to changes in ambient temperature and pressure. Such expansion and contraction of air may cause the ink to leak from the writing tip of the pen.
Other such known pens include a buffer for storing excess ink in response to changes in ambient temperature and pressure. The excess ink is typically stored in the front of the buffer near the tip of the pen (i.e., due to gravity). However, such known pens have several disadvantages: the ink capacity of the buffer is limited such that when the buffer is full the excess ink leaks from the pen, and the ink is often permanently stored in the buffer resulting in decreased buffer capacity and wasted ink. Another of such known pens provides for the cleaning of ink from the buffer when the pressure inside the pen is increased by venting air into the pen through an external vent. Such known pens, however, only clean a small portion of the buffer.
Accordingly, it would be advantageous to provide a hydrostatically stable pen that responds to repeated temperature and pressure changes by reducing the accumulation of ink in the buffer without substantially leaking or dripping. It would also be advantageous to provide a pen that optimizes the efficiency of the buffer by purging the buffer during changes in ambient temperature or pressure. It would also be advantageous to provide hydrostatic stability when the pen is oriented in any direction. Other advantages of the subject matter recited in the appended claims will become apparent to those skilled in the art upon review of the specification and the claims.